An ordinary magnetic rotation-angle detector detects a rotation angle of a rotary drum as a rotation angle of a body of revolution such as a motor coupled to the rotary drum via a shaft by using a result of detection by a magnetic sensor with respect to a magnetic field generated by a multi-pole magnetic pattern on the rotary drum having N and S poles which are provided on an outer circumference and on which magnetization alternately occurs at a magnetization pitch λ. Magnetic resistance elements (MR elements) such as AMR elements are used for the magnetic sensor, and the magnetic sensor uses properties of the AMR elements that electric resistance changes in proportion to the change in a magnetic field. The magnetic sensor causes the MR elements to detect the changes in the magnetic field due to the rotation of the rotary drum, and detects the rotation angle of the rotary drum based on a sine-wave detection signal obtained by this detection of the change.
More specifically, the magnetic sensor is configured to output a sine-wave detection signal (a phase-A detection signal) and a cosine-wave detection signal (a phase-B detection signal) in phases shifted by 90 degrees from each other by arranging the MR elements in an array. A computing device that receives these detection signals from the magnetic sensor and that is located in rear of the magnetic sensor performs an arc tangent arithmetic operation between the phase-A detection signal and the phase-B detection signal, thereby computing the rotation angle of the rotary drum.
Generally, when a magnetic flux density waveform is converted into a detection signal (an MR waveform) of the MR elements, harmonic distortions are always superimposed on the MR waveform for the following reasons. An MR characteristic curve used to convert a magnetic flux density into the detection signal (a resistance change rate) of the MR elements is of a shape of a quadratic function having a minimum at the position of a magnetic flux density of zero. Furthermore, as for higher magnetic fields (magnetic flux densities) than an inflection point (deviating from the quadratic function shape), outputs from the MR elements saturate. Therefore, the MR waveform becomes closer to a rectangular waveform.
Patent Literature 1 describes a configuration in which a plurality of MR elements are arranged so that the MR elements other than one MR element are away from the one MR element by λ/10, λ/6, and λ/6+λ/10, respectively in a magnetic sensor for detecting the position of a rotary drum on which magnetic signals are recorded at pitches λ. This configuration has the following effects. According to Patent Literature 1, a differential amplifier located in rear of these four MR elements adds up output signals from the MR elements. Therefore, it is supposed that it becomes possible to cancel fifth and third harmonic waves and to cancel waveform distortions of an output signal from the magnetic sensor generated by the saturation of the MR elements.